The primary objective of this research is to identify the contribution of obesity to international differences in longevity. Adults in the United States have a higher prevalence of obesity than adults in any other country in Europe, North America, or East Asia. At the same time, life expectancy in the United States has fallen below that of most other OECD countries and ranked 32nd in the world in 2008. One of the prime candidates to account for the US disadvantage in health and longevity is its high level of obesity. A key input to evaluating the contribution of obesity to international differences in longevity is the set of individual-level mortality risks associated with different levels of obesity. This project will identify the set of mortality risks associated with obesity that should be used in international and intertemporal comparisons. We do so by explicitly introducing two other factors on which the mortality risks associated with current obesity depend: an individual's history of obesity and his or her smoking status. The omission of obesity histories from earlier studies has led to biased estimates of the effects of current obesity. Non-smokers show a higher mortality risk from obesity than smokers, so that countries that smoke heavily should be expected to have lower risks from obesity. We will apply these sets of obesity risks to calculate population attributable risks and obesity's implications for survivorship and longevity. An important product of our work will be an explanation of the large declines that have been observed in the estimated effect of obesity on mortality. In addition, we will develop a new indirect approach to estimating the contribution of obesity to international differences in longevity by building on previous methods developed to estimate smoking-attributable mortality. Epidemiologic and demographic studies of obesity would benefit greatly from a clarification of the mortality risks associated with obesity. Such a clarification would also contribute to improved projections of mortality in the US and elsewhere. Uncertainty about future of mortality is the single factor to which fiscal balances in the Social Security Trust Fund are most sensitive. The contribution of this project to improved projections is enhanced by its explicit incorporation of obesity histories into the risk analysis, since these histories are revealed well in advance of actual mortality conditions.